Method for tightening screw joints

ABSTRACT

A method for tightening screw connections in series assembly. The method includes continuously monitoring a torque and a rotational angle of the screw. Initially, the screw is tightened by turning it in a rotational direction through a first rotational angle by exerting a desired torque. The screw is then released by turning the screw in a reverse rotational direction through a particular rotational angle. The screw is then re-tightened by turning it in the rotational direction through a second rotational angle by exerting the desired torque. A first absolute difference in rotational angles between the first rotational angle and the second rotational angle is determined and the value for the first absolute difference in rotational angles is then utilized to provide a monitoring of the tightening characteristics in the screw. The conditions during the tightening and the releasing of the screw is thereby taken into consideration during the method of the present invention to determine the seating and tightening characteristics in the screw.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for tightening screw connections inseries assembly, in which a screw connection, in each case monitoredwith regard to torque and rotational angle, is tightened for a firsttime by a desired moment, then is released again over a certainrotational angle and is tightened for a second time by a desired moment.

In order to tighten screw connections in series assembly methods areknown which are essentially controlled with respect to torque and/orwith respect to rotational angle. Using these known methods the desiredscrew prestressing force of the particular screw connection is achievedmore or less precisely, the said force being described as a parameterfor each screw connection. However, a constant, reproducible quality ofthe screw connection, i.e. a uniform screw prestressing force duringeach tightening, cannot be achieved, since the screw prestressing, forceobtained fluctuates considerably on account of a multiplicity of factorsdetermining it. For example, oil and dirt in the thread effect thecoefficient of friction which, in turn, has a direct effect on the screwprestressing force obtained with a prescribed torque. Furthermore, theproperties of the tightening tool also have an effect on the resultingscrew prestressing force, since, for example, when a motor is used, thelatter when switched off still continues to rotate the screw to betightened somewhat further because of the inertia of the masses involvedand causes a higher screw prestressing force than desired.

In the case of the tightening method which is controlled with respect totorque and is based on the assumption that the prestressing force actingon the screw is proportional to the torque applied, the switching offpoint of the screwing procedure is prescribed by a defined measuredvalue for the torque. In order to be able to arrive at this prescribedvalue for the torque during the tightening in a targeted manner, theprofile of the torque can be monitored during the tightening, as isdescribed, for example in U.S. Pat. No. 5,591 919, or the screwconnection can be tightened in three stages using decreasing rotationalspeeds in each case, as is described in U.S. Pat. No. 5,062,491. It isalso known, in order to shorten the time required for the clamping, toacquire parameters during preceding learning cycles in order thereby tobe able to allow the motor to run at a high speed during tightening ofthe screw connections and to be able to stop it exactly when the desiredtorque is reached, as is described in European Patent Document EP 753377 A1 corresponding to U.S. Pat. No. 5,650,574). Conversely, it islikewise known, in order to extend the time taken for the screwingprocedure, to provide the tool with a spring which is arranged betweenthe driving element and the gripping element of the tool, as isdisclosed in German Patent Document DE 32 10 929 A1 corresponding toU.S. Pat. No. 4,463,293). In order to avoid vibrations of the springhere, at a prescribed tightening torque the motor is switched to run inreverse, in order to slacken the spring, then is switched off andswitched briefly again into the original direction of rotation. Finally,German Patent Document DE 35 21 937 A1 discloses a tightening methodwhich is controlled with respect to torque and in which the torquerequired for reaching the prescribed prestressing force can bedetermined free of frictional forces. For this purpose, before thetightening point is reached the screw connection is reversed over acertain rotational angle and, when a defined rotational angle isreached, is again tightened up to the tightening point. The clampingforce applied to the screw is calculated free of frictional forces fromthe difference between the tightening torque and the counter torque inthe defined rotational angle.

The tightening method which is controlled with respect to torque isbased on the fact that when the screw head is screwed on and thecomponents joined together, the extension of the screw and therefore theaxial force produced in it is proportional to the angle over which thescrew is rotated. Thus, for example, DE 42 14 354 A1 discloses atightening method which is dependent on torque and rotational angle andin which during the tightening of the screw connection first of all thejoining point is determined and then from this joining point theswitching off point is determined either in accordance with prescribedvalues for the torque or in accordance with prescribed values for therotational angle. It is known from U.S. Pat. No. 5,284,217 to monitorthe torque and the rotational angle during the tightening in orderthereby firstly to determine the required torque in order to reach aprescribed prestressing force and secondly to check whether a prescribedtolerance zone with regard to the torque and the rotational angle isbeing maintained. Finally, it is also known, in particular in the caseof components which are relevant for safety, after the tightening torelease the screws over a defined angle and to tighten them again by theprescribed desired moment. In this case, the torque is plotted over therotational angle and the shape of the torque profile is used for anassessment with regard to the quality of the screw connection and causesof error (Maschinenmarkt, Würzburg 102 (1996) 20).

As already mentioned, using these known tightening methods it ispossible, more or less precisely, to reach the required prestressingforce of the screw connection, essentially by detecting the torque androtational angle. However, the extent to which this prestressing forceobtained is reduced during operation of the screw connection by seatingphenomena cannot be assessed using this method. Seating phenomena occurin screw connections due to the material creeping as a consequence ofthe tightening and lead to an automatic loosening or even release of thescrew connection due to the prestressing force being reduced.

Starting from this background, it is therefore the object of the presentinvention to provide a method for tightening screw connections in seriesassembly, in which a screw connection, in each case monitored withregard to torque and rotational angle, is tightened for a first time bya desired moment, is then released again over a certain rotational angleand is tightened for a second time by a desired moment, it beingpossible for seating phenomena in screw connections to be recognizedusing this method.

This object is achieved by the absolute difference in rotational anglebetween the first and the second tightening being determined, and by themagnitude of this amount of difference being used as a process-relevantparameter for monitoring purposes.

This determined absolute difference in rotational angle between thefirst and the second tightening is a gauge for seating phenomena as mayoccur in screw connections due to various reasons. By this means,seating phenomena in screw connections can easily be recognized even asthe screw connection is being tightened, and by means of the comparisonof the amount of difference determined with a prescribed tolerancevalue, the security of the screw connection can be significantlyincreased and therefore its quality as a whole improved.

If the absolute difference in rotational angle determined lies below aprescribed minimum difference in angle or above a prescribed maximumdifference in angle, an impermissible angular offset exists which, apartfrom the assessment of seating phenomena or the seating extent, can alsosupply conclusions as to possible sources of error in the screwconnection. According to a particularly preferred embodiment, theabsolute difference in rotational angle is therefore compared with aprescribed minimum and maximum difference in rotational angle and anerror message is produced if the difference in rotational angledetermined lies above or below these prescribed values of the differencein angle.

These two additional parameters can be stored easily and without a largeoutlay in process control systems of existing assembly lines, with theresult that the method according to the invention can be integrated in asimple manner in existing series assemblies. When there is an errormessage the screw connection is advantageously passed on to refinishing.In refinishing the rejected screw connection can be released andinvestigated and the cause of the error message eliminated. However, itis also possible to select the rejected screw connections in accordancewith the type of error message in each case and, for example when theprescribed maximum difference in angle is exceeded, to pass the screwconnection onto a second tightening procedure, which is provided inaccordance with a further preferred embodiment of the invention. Theabsolute difference in rotational angle determined during the secondtightening procedure is compared with the value determined during thefirst tightening procedure and an error message is produced if, withregard to this value, the second difference in rotational angledetermined lies above or below the prescribed maximum or minimumdifference in rotational angle. When there is an error message the screwconnection is advantageously passed onto refinishing, since the causecould not be eliminated using the tightening procedure again and has tobe investigated more precisely. If an error message is not produced, thecause for the error message during the first tightening/releaseprocedure has been eliminated during the second tightening/releaseprocedure, and because the prescribed tolerance value is maintained thescrew connection can remain in the series assembly;

Although the absolute difference in rotational angle can be determinedin any desired manner, provided that it is thereby possible to use themethod in the series assembly, it is, however, of advantage if in orderto determine the absolute difference in rotational angle, the torqueapplied is plotted over the actual rotational angle, so that arepresentation in loop form is produced. In this representation, thedifference in rotational angle between the first tightening and thesecond tightening can then be recognized in a simple manner, so that inaccordance with an advantageous embodiment, the difference in angle isdetermined from the looped representation of the torque/rotationalangle.

In a representation of this type, compared to the customary continuousrepresentation of torque and rotational angle, for one thing the forwardrotation and reverse rotation of the screw connection can be readilyrepresented by plotting the actual rotational angle and therefore itsactual increase or decrease in size. For another thing, and moreessential for the method according to the invention, the difference inrotational angle between the first and the second tightening can betaken directly from a representation of this type without deviations.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described in greater detail below with reference to thedrawing, in which:

FIG. 1 illustrates a graphical representation of the screw tighteningmethod of the present invention; and

FIG. 2 illustrates a screw connection with which a method according tothe present invention is useable.

DETAILED DESCRIPTION OF THE DRAWING

In FIG. 1, the looped representation of torque/rotational angle can beseen, the torque Nm being plotted over the actual rotational angle α.During the tightening of a screw connection 4, represented in FIG. 2,this results in the looped curve 1 which is represented and in which itcan readily be recognized that the screw connection is initiallytightened up to the desired moment 3, then the torque is reversed andthe screw connection is released in order finally to be tightened againup to the desired torque 3. The difference A in rotational angleproduced in this case can clearly be recognized on account of the formof representation selected, and can be used for monitoring purposes.

Using the method described here, the overall process of thetightening/release procedure is taken into account and inclusion of thedifference in rotational angle as an additional process-monitoringparameter enables seating phenomena in screw connections to berecognized even as tightening is taking place.

What is claimed is:
 1. A method for tightening a series of screwconnections in which each screw connection is monitored with regard totorque and rotational angle comprising: tightening one of the screwconnections for a first time through a first rotational angle and up toa desired moment, releasing said one of the screw connections over acertain rotational angle, tightening the one of the screw connectionsfor a second time through a second rotational angle and by a desiredmoment, determining an absolute difference between the first and secondrotational, and comparing said absolute difference with prescribedminimum and maximum differences for monitoring purposes.
 2. The methodaccording to claim 1, and further comprising producing an error messageif the absolute difference lies above the prescribed maximum differenceor below the prescribed minimum difference.
 3. The method according toclaim 2, and further comprising passing the one of the screw connectionson for refinishing when there is an error message.
 4. The methodaccording to claim 2, and further comprising releasing the one of thescrew connections for a second time and tightening the one of the screwconnections for a third time when there is an error message and theprescribed maximum difference has been exceeded, comparing an absolutedifference determined in this case with the absolute difference betweenthe first and second angles, and producing an error message if theabsolute difference determined in this case lies above the prescribedmaximum difference or below the prescribed minimum difference.
 5. Themethod according to claim 4, and further comprising passing the screwconnection on for refinishing when there is an error message.
 6. Themethod according to claim 1, wherein, in order to determine the absolutedifference, a torque applied is plotted during tightening and releasingso that a representation in loop form is produced.
 7. The methodaccording to claim 6, wherein an impermissible angular offset fallingbelow a minimum difference in angle or exceeding a maximum difference inangle is determined from the representation in loop form produced.
 8. Amethod for tightening a screw connection comprising: continuouslymonitoring a torque and a rotational angle of the screw; tightening thescrew by turning it in a rotational direction through a first rotationalangle by exerting a desired torque; releasing the screw by turning thescrew in a reverse rotational direction through a particular rotationalangle; re-tightening the screw by turning it in the rotational directionthrough a second rotational angle by exerting the desired torque;determining a first absolute difference in rotational angles between thefirst rotational angle and the second rotational angle; and comparingthe first absolute difference with prescribed minimum and maximumdifferences to monitor tightening characteristics in the screw.
 9. Themethod of claim 8, further including producing an error message if thefirst absolute difference in rotational angles lies above thepredetermined maximum difference or below the predetermined minimumdifference in rotational angles.
 10. The method of claim 9, and furthercomprising passing the screw on to a refinishing state if the errormessage is produced.
 11. The method of claim 9, and further comprisingreleasing the screw again and then re-tightening the screw by turningthe screw in the rotational direction through a third rotational angleby exerting the desired torque when the first absolute difference inrotational angles lies above the maximum difference in rotational anglesand an error message is produced, determining a second absolutedifference in rotational angles between the first rotational angle andthe third rotational angle, and producing a second error message if thesecond absolute difference in rotational angles lies above thepredetermined maximum difference or below the predetermined minimumdifference in rotational angles.
 12. The method of claim 11, and furthercomprising passing the screw on to a refinishing stage if the seconderror message is produced.
 13. The method of claim 8, whereindetermining the first absolute difference in rotational angles includesplotting the torque over the rotational angle and thereby producing alooped representation of torque/rotational angle.
 14. The method ofclaim 13, further including using the looped representation of thetorque/rotational angle to determine if the first absolute difference inrotational angles lies above or below the predetermined minimum andmaximum differences in rotational angles.